Such a valve assembly is disclosed in International Patent application no. WO03/031855, and comprises a valve body in which are mounted two pressure responsive pistons, one of which is mounted inside the other, and two return springs, one of which extends between the valve body and the outer piston, and the other of which is located in a chamber inside the outer piston and extends between the outer and inner pistons. An axially extending central aperture is provided through both pistons to allow flow of fluid from a valve assembly inlet into the chamber inside the outer piston and above the inner piston, and into a control chamber above the outer piston.
The valve assembly is of relative complex construction, and the parts must be manufactured to a close tolerance to ensure that fluid tight seals can be provided where required whilst allowing the necessary movement of the two pistons, in particular the two pistons must be manufactured to a close tolerance to ensure that a fluid tight seal is provided between the inner and outer pistons whilst allowing relative movement between the two pistons. Moreover, the presence of two return springs in the valve assembly further complicates manufacture of the valve assembly, as one return spring must be inserted into the chamber between the two pistons, and performance of the valve assembly depends partly on the performance of two return springs. In addition, maintenance of the valve assembly is inconvenient because the chamber inside the outer piston in which one of the return springs is located cannot be accessed, for example to check that the seal between the two pistons is adequate and sufficiently well lubricated, or to check or replace the return spring, without dismantling the entire valve assembly.
It is specified that the valve assembly described in WO03/031855 is suitable for use as either an ABS or and EBS valve in a vehicle brake system, but this valve assembly is, in fact, more suitable for use as an ABS valve than as an EBS valve.
In normal use, an ABS valve is passive and allows flow of fluid from the valve inlet to the vehicle brake, i.e. is in the apply state. Only if wheel lock is detected is the ABS valve assembly activated to either release fluid pressure from the vehicle brake or to hold fluid pressure in the vehicle brake. The valve assembly disclosed in WO03/031855 is in the apply state when the solenoid valve is de-energised.
In contrast, an EBS valve assembly must normally be in the release state, i.e. must release fluid from the vehicle brake and block flow of fluid from a pressurised fluid reservoir through the valve inlet. Only when a driver of the vehicle demands braking is the EBS valve assembly operated to allow flow of fluid from the pressurised fluid reservoir to the vehicle brake. The valve assembly disclosed in WO03/031855 is in the release state when the solenoid valve is energised, and therefore electrical power consumption and wear on the solenoid valve would be excessive if this valve assembly were to be used as an EBS valve.